The invention relates to a method for automatic braking of motor vehicles with an anti-lock brake system, such as is disclosed, for example, in German patent document DE 40 28 290 C1.
In the latter publication, it is proposed to trigger automatic braking when the actuation speed of the brake pedal exceeds a threshold value. The automatic braking process is carried out until a termination criterion is fulfilled. One possible termination criterion, for example, is that the pedal retraction speed exceeds a threshold value. During automatic braking, a higher brake pressure is produced than that which corresponds to the position of the brake pedal.
The execution of automatic braking is associated with the fulfillment of the triggering criterion. The purpose of automatic braking is to shorten the braking (stopping) distance in critical driving situations. Braking operations in which the driver desires such a strong deceleration that the wheels are inclined to lock occur only in critical driving situations in which either there is only low adhesion between wheel and road, or the braking distance available is estimated by the driver as hardly sufficient.
In braking operations, when an anti-lock brake system commences control of the brake pressure of an axle, many drivers do not increase the brake pressure further, since they assume that the maximum possible brake pressure has already been produced. Thus, the maximum possible brake pressure is achieved only for the axle at which the brake pressure has started to be controlled by the anti-lock brake system, and a longer braking distance is required than would be for optimum braking by the driver.
The object of the present invention is to carry out an automatic braking process when a critical driving situation is detected by virtue of brake pressure being controlled by the anti-lock brake system. The degree of wheel slip at which the control of the brake pressure by the anti-lock brake system starts is referred to in this application as the locking limit.
This object is achieved by the automatic braking process according to the invention in which automatic braking is triggered when it is detected by the anti-lock brake system that at least one wheel of an axle of the vehicle has reached the locking limit. At this time, the anti-lock brake system commences control of the brake pressure for the wheels of this axle, so that an increase in brake pressure at the wheels affected by the ABS control is not possible. Because of the axle load distribution of the vehicle and because of the distribution of braking force at the wheels of the other axles, however, the locking limit of the latter wheels is not yet reached. According to the invention, therefore, the brake pressure applied to these wheels is further increased during the automatic braking process.
According to one embodiment of the invent ion, the brake pressure is increased until the locking limit of all wheels is reached, and thus the brake pressure for each axle is controlled by the anti-lock brake system.
A further embodiment of the invention proposes that the automatic braking process not be triggered until the ABS control of an axle has been operating for at least a time t0, which is between 0.1 s and 1 s, preferably for 400 ms.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.